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Oxidative stability of edible argan oil: A two-year study
LWT - Food Science and Technology 44 (2011) 1e8
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Oxidative stability of edible argan oil: A two-year study Saïd Gharby a, b, Hicham Harhar a, Dominique Guillaume c, *, Aziza Haddad b, Bertrand Matthäus d, Zoubida Charrouf a a
Laboratoire de Chimie des Plantes et Synthèse Organique, Département de Chimie, Faculté des Sciences, Université MohammedV-Agdal, BP1014, Rabat, Morocco Laboratoire Contrôle Qualité, Lesieur-Cristal, 1 rue Caporal Corbi, 20300 Roches Noires-Casablanca, Morocco c CNRS-UMR6229, Université de Reims-Champagne Ardenne, 51 rue Cognacq Jay, 51100 Reims, France d Max Rubner-Institute, Federal Research Institute for Nutrition and Food, Department for Lipid Research, Piusallee 68/76, 48147 Münster, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 March 2010 Received in revised form 31 May 2010 Accepted 2 July 2010
The present study investigated the oxidative stability of the threemarketed types of edible argan oil. Edible argan oil is prepared by pressing the slightly roasted kernels of peeled argan fruit. High quality edible argan oil is exclusively prepared using mechanical presses. However, hand-extracted argan oil is still artisanally produced and can be found in local markets. In this latter case, goat-peeled fruit is still sometimes introduced in the oil production chain even though the resulting oil is notoriously of unsatisfactory quality. The oxidative stability of press-extracted, hand-extracted, and goat-peeled fruit derived argan oil was analyzed using as physicochemical metrics: fatty acid composition, b-carotene level, phosphorus level, tocopherol level, iodine index, saponification, peroxide and acid values, specific extinction, and Rancimat induction time. The variations of these parameters were evaluated over a period of 2 years at 5 C, 25 C (protected or exposed to sunlight), or 40 C. After this period of time, mechanically extracted argan oil still presents an excellent physicochemical profile. Domestic and traditionally prepared argan oil presents much less satisfactory properties after the same period. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Argania spinosa Edible argan oil Long-term oil preservation Long-term oil quality
1. Introduction The main consequence of the drastic improvements recently brought to argan oil preparative process (Charrouf, Guillaume, & Driouich, 2002) was its entry as a major actor in the edible-andexpensive-oil closed circle. Indeed, argan oil that was almost unknown out of the limits of the argan forest twenty years ago is now sold in virtually every gourmet-stores around the world. Edible argan oil is the basis of the Amazigh diet (Charrouf & Guillaume, 2010). It is prepared from the roasted kernels of the fruit of the argan tree (Argania spinosa (L.) Skeels) that is exclusively endemic in Southern Morocco (Morton & Voss, 1987). Recent attempts to sustainably develop this region (Charrouf & Guillaume, 2009) threatened by desertification have been consecutive to its designation as a Biosphere Reserve by UNESCO in 1998 and to our early intensive chemical work principally carried out on argan tree metabolites (Charrouf & Guillaume, 2002; Guillaume & Charrouf, 2005) and its fruit-derived oil (Charrouf & Guillaume, 1999). Some of these efforts have been fruitful and have led to the implantation in the argan forest of several argan oil-producing
* Corresponding author. Tel.: þ33 326918473; fax: þ33 326918029. E-mail address: [email protected] (D. Guillaume). 0023-6438/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2010.07.003
woman cooperatives where high quality argan oil is now prepared using a strictly controlled process. Particularly, new rules include 1) the banishment of goat-peeled fruit, and therefore the necessary use of scratching machines to peel the argan fruit, and 2) the use of screw-presses to extract the oil in place of water-requiring hand malaxing of argan dough. Consequently, low-grade argan oil has been gradually replaced by high quality argan oil. The combination of high levels of unsaturated fatty acids and antioxidants, and its unique taste and pharmacological properties (Charrouf & Guillaume, 2008), have ultimately boosted high quality argan oil market share. If the negative influence of hand-malaxing, uncontrolled kernelroasting time, or “goat-peeling” on argan oil quality is now particularly well documented (Charrouf, El Hamchi, Mallia, Licitra, & Guillaume, 2006; Hilali, Charrouf, El Aziz Soulhi, Hachimi, & Guillaume, 2005), the real improvement in terms of oil preservation time has not been precisely studied and quantified, yet. To fill this gap, we decided to evaluate over a two year period the oxidative stability of argan oil prepared 1) traditionally (hand malaxed), 2) mechanically (screw-pressed), and 3) using goatpeeled fruit (hand malaxed and animal-peeled). Such a study is highly desirable to determine an accurate shelf life for each type of oil on the domestic or international market. During our study, oil samples were kept at 5 C, 25 C, or 40 C. Because oil samples
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refrigerated at 5 C or heated at 40 C were necessarily sunlight protected (fridge or oven; respectively), and since light is wellknown to possibly influence edible oil oxidation (Cinquinta, Esti, & La Notte, 1997), we also decided to evaluate the influence of light on our oil samples kept at 25 C by use of clear glass or dark-glass bottles. Therefore, twelve oil samples were periodically analyzed over a two year period. To be able to eventually link possible characteristic variations to a preparative process, all our studied oil samples were prepared in the same cooperative. Consequently, to ascertain the general character of our results, before the beginning of our study, we first demonstrated the lack of influence of the geographic origin on argan oil initial physicochemical parameters by comparing the oxidative stability of argan oil produced at three different locations of the argan forest. 2. Materials and methods 2.1. Chemicals and materials All the reagents were of analytical or HPLC grade. Isooctane and isopropanol used as HPLC mobile phase and cyclohexane used for extinction coefficient determination were purchased from Professional Labo (Casablanca, Morocco). Clear- and brown-glass bottles were purchased from Cfimu Sarl (Casablanca, Morocco). 2.2. Sample collection Argan oil samples analyzed to determine the initial oxidative parameters were prepared in 2006 in the woman cooperatives of Ait Baha (Chtouka-Ait Baha county, Morocco), Tidzi (Essaouira county, Morocco), and Tiout (Taroudant county, Morocco) following our previously reported protocol (Hilali et al., 2005). For the twoyear study, argan oil samples were those prepared in the woman cooperative of Tiout. 2.3. Sample distribution For the determination of the initial physicochemical parameters of argan oil, three types of oil were prepared: artisanally extracted argan oil (AAO), mechanically extracted argan oil (MAO), and traditional artisanal argan oil obtained from goat-peeled fruit (GPAO). Oil samples prepared from Ait Baha, Tidzi, and Tiout are indexed AB, TZ, and TT; respectively. Time-dependent oxidative stability was studied by comparing the physicochemical properties of twelve samples. Ten liters of AAO, MAO, and GPAO were prepared. Each oil type was distributed in 360 60-mL-glass bottles: 270 clear- and 90 brown-glass bottles. The remaining oil was used to determine initial values. For a given oil type, 30 clear-glass bottles were stored at 5 C, 25 C, and 40 C. Additionally, 30 brown-glass bottles were stored at 25 C. Headspace volume (bottleneck volume) for each bottle was 3.5 (0.5) mL. 2.4. Analytical methods Samples stored at 5 C were analyzed after 5, 11, 17, and 23 months of storage. Samples stored at 25 C or 40 C were analyzed after 1 month of storage then every 2 months until month 23. Acid value, peroxide value, saponification value, iodine index, and UV-light absorption (K270 and K232) were determined as previously described (Hilali et al., 2005). For the fatty acid composition determination, the methyl esters were analyzed on a CP-Wax 52CB column (30 m 0.25 mm i.d.) using helium (flow rate 1 mL/mn) as a carrier gas. Initial oven temperature
was set at 170 C; injector temperature 200 C; detector temperature 230 C. Injected quantity was 1 mL for each analysis. The oxidative stability of each sample was determined as the induction period (IP, hours) recorded by a Rancimat 743 (Metrohm) apparatus using 3 g of oil sample with an air flow of 20 L/h. To identify the initial oxidative parameters, oxidative stability was successively determined at 90 C, 100 C, 110 C, 120 C, 130 C, and 140 C. For the two-year study, IP was determined at 110 C. Sterol composition was determined after trimethylsilylation of the crude sterol fraction. Trimethylsilylated derivatives were analyzed by gas chromatography using a Varian 3800 instrument equipped with a VF-1ms column (30 m 0.25 mm i.d.) using helium (flow rate 1.6 mL/mn) as carrier gas. Column temperature was isothermal at 270 C, injector and detector temperature was 300 C. Injected quantity was 1 mL for each analysis. Individual tocopherol content was determined on the basis of the AOCS Official method Ce 8-89 (American Oil Chemists’ Society,1993). Tocopherols were analyzed by HPLC using Shimadzu instruments equipped with a C18-Varian column (25 cm 4 mm). Detection was performed using a fluorescence detector (excitation wavelength 290 nm, detection wavelength 330 nm). Eluent used was a 99:1 isooctane/isopropanol (V/V) mixture, flow rate 1.2 mL/mn. Phosphorus content was determined using the NF T60-227 recommendation (Paquot & Hautfenne, 1987). b-Carotene content was determined using a PFX-995 lovibond tintometer (cell length 10 mm). 2.5. Statistical analysis Values reported in tables and figures are the means SE of 3e5 replications. The significance level was set at P ¼ 0.05. Separation of means was performed by Turkey’s test at the 0.05 significance level. 3. Results and discussion Genuine edible argan oil is exclusively prepared in Morocco since argan trees are only endemic in this country. Three types of edible argan oil can be found on the market: “certified”, “artisanal”, and “family”. Those denominations reflect the oil preparative process. Certified argan oil is exclusively prepared in woman cooperatives by use of mechanical presses; it is sold on both domestic and international markets. Artisanal argan oil is prepared by decanting the liquid resulting from the prolonged mixture of argan dough and water; it is mainly sold on Morocco domestic market but can also be purchased on the Internet. Family argan oil is generally prepared under rudimentary conditions with the risk of bacteriologically unsafe water and use of goat-peeled fruit; it is used in the family circle but surplus of oil is sometimes sold on the local market. Each type of oil presents its own physicochemical (Hilali et al., 2005) and organoleptic profile (Matthäus et al., 2010). The aim of our study was to evaluate the influence of a prolonged storage on argan oil physicochemical properties and oxidative stability. Consumption of argan oil usually occurs within 18e24 months. Accordingly, we chose a maximum storage time of two years for our study. Variations in oil processing can influence the initial oxidation of edible oil (Tatum & Chow, 1992). Therefore, we decided to determine the oxidative properties of the three common types of edible argan oil: mechanically-extracted (MAO), artisanally-extracted (AAO), and handextracted using goat-peeled fruit (GPAO). Four different storage conditions: refrigerated at 5 C, 25 C light-unprotected, 25 C lightprotected, and 40 C (oven) were considered. Our study began with the careful determination of the initial parameters of our samples. Due to the large area covered by the argan forest, the question of the incorporation of the oil geographic origin as a to-be-considered parameter came out rapidly. Consequently, we first decided to carry
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Table 1 Physicochemical parameters of argan oil prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goatpeeled fruit (GPAO). Oil samples prepared from Ait Baha, Tidzi, and Tiout are indexed AB, TZ, and TT; respectively. Mean standard deviation of the values (five replicates) are presented. MAOTZ
MAOAB Acid Value (mg/g) Peroxide value (Meq/kg) Moisture (mg/100 mg) K232 K270 b-carotene (ppm) Phosphorus (mg/103 g) Phospholipid (mg/100 mg) SFAa (mg/100 mg) UFAa (mg/100 mg) a
0.3 0.7 0.07 1.44 0.25 20 42.8 0.3 19 80
0.05 0.1 0.01 0.06 0.05 0.5 0.2 0.7 1
0.3 1.2 0.06 1.18 0.2 20 61.5 0.2 17.7 82
MAOTT 0.05 0.1 0.01 0.07 0.05 0.5 0.8 0.5 0.5
0.3 0.6 0.05 1.02 0.18 21 80.2 0.25 19.3 80
AAOAB 0.02 0.1 0.01 0.06 0.05 0.5 0.8
0.3 0.5
0.6 1 0.08 1.28 0.19 11 7.8 0.02 19.1 81
AAOTZ 0.1 0.1 0.01 0.06 0.05 0.5 0.1
0.3 0.5
0.5 1 0.06 1.21 0.18 13 5.3 0.01 18.2 81
AAOTT 0.1 0.1 0.02 0.06 0.05 0.5 0.1
0.1 0.5
0.3 1 0.08 1.24 0.22 18 3.9 0.01 19.6 79
GPAOAB 0.1 0.2 0.01 0.06 0.05 0.5 0.1
0.1 0.5
0.3 1.1 0.06 1.29 0.21 15 3.6 0.01 20 78.5
0.1 0.1 0.01 0.07 0.05 0.3 0.2
0.4 0.5
GPAOTZ 1.1 1.3 0.25 1.49 0.18 16 9.1 0.03 17 82
0.1 0.1 0.01 0.06 0.05 0.5 0.1
0.2 0.5
GPAOTT 0.7 1.5 0.09 1.37 0.17 17.5 6.1 0.02 18 80.5
0.1 0.2 0.01 0.06 0.05 0.5 0.1
0.2 0.2
SFA: saturated fatty acids, UFA: unsaturated fatty acids.
out an oxidative stability study/physicochemical analysis of argan oil samples coming from the three main locations, in terms of argan oil production, of the argan forest. 3.1. Determination of the initial physicochemical parameters of the oil samples Some physicochemical parameters of freshly prepared argan oil have already been reported to be either poorly dependent or independent on the nut harvest location (Cayuela et al., 2008; Charrouf et al., 2006). Because 1) short term autoxidation of argan oil has been only partially studied (Chimi, 2005; Chimi, Cillard, & Cillard, 1994), 2) the aspect of preservation has never been investigated in previous studies, 3) equipments used to obtain mechanically extracted argan oil are regularly upgraded, and 4) minute traces of metals can modify oil quality (Marfil et al., 2008), we decided to examine the physicochemical parameters of argan oil samples coming from Ait Baha (AB), Tidzi (TZ), and Tiout (TT), the three largest argan oil woman cooperatives in the argan forest. Table 1 lists the results of the physicochemical parameters analyzed. Satisfactorily, all fresh argan oil samples displayed the physicochemical properties necessary to access the edible grade as defined by the recommendations of the official argan oil norm (Service de normalisation industrielle, 2003). Nevertheless, the high acid value of GPAOTZ (>0.8%) was incompatible with an “extra virgin” label, even though it was acceptable for a “pure virgin” grade (Service de normalisation industrielle, 2003). Comparison between the determined parameters indicated that MAO consistently contained a significantly higher level of phospholipid/phosphorus than AAO and GPAO. It is likely that the development of heat at the press head during mechanical extraction results in a transfer of phospholipids into the oil and hence to a high amount of phospholipids in MAO. The low amount of phospholipids in AAO and GPAO results from a poor phospholipid extraction at room temperature. Phospholipids can trigger technical problems during oil degumming or refining. In our case, such a consideration is only of limited importance since, oppositely to cosmetic argan oil, edible (extra) virgin argan oil is not refined. More importantly, phospholipids can act as antioxidants or prooxidants (Choe & Min, 2006; Koprivnjak et al., 2008) depending on their concentration and the
presence of metal ions (Choe & Min, 2006) or tocopherols (Judde, Villeneuve, Rossignol-Castera, & Le Guillou, 2003; Koga & Terao, 1995). Since argan oil is notoriously rich in tocopherols, the high content in phospholipids in MAO, compared to AAO and GPAO should merit further attention. Other studied parameters ((un) saturated fatty acid, b-carotene level, UV absorption) were remarkably constant, any of them significantly varying as a function of the oil geographic origin. 3.2. Determination of the initial oxidative stability of the oil samples Conversely to argan oil physicochemical parameters, its oxidative stability has never been studied as a function of the oil geographic origin. To get a complete picture of argan oil oxidative stability, we decided to determine the induction period by Rancimat test at 90, 100, 110, 120, 130, and 140 C of our oil samples prepared in the three different locations. Results obtained at 110 C are presented Table 2. Interestingly, homogenous Rancimat induction periods were observed within each group. For every given temperature, MAO consistently and independently of the geographic origin displayed much longer rancimat induction periods than AAO or GPAO. Oxidative stability of argan oil has been attributed to its high content in tocopherols (Rahmani, 2005), and carotenes (Collier & Lemaire, 1974). Since these families of components are in a similar quantity in the three types of oil (Hilali et al., 2005), our results show that in fresh argan oil, phospholipids do not act as prooxidants, and presumably act as antioxidants, reinforcing the strong preservation activity of tocopherols. Notably large induction period differences between the MAO group and the AAO and GPAO groups were observed between 90 and 110 C. At 90 C, MAO average rancimat induction period was found to be 110 6 h, whereas it was only 76 6 h, and 70 2 h for AAO, and GPAO, respectively. At 110 C, the difference between the mean rancimat induction period of MAO and AAO or GPAO was 11 2 h. Unexpectedly, AAO and GPAO displayed similar rancimat induction periods even though GPAO preservation time is commonly said to be low. This apparent contradiction can be explained since in our study AAO and GPAO samples were prepared using bacteriologically safe water, a parameter never controlled when argan oil is
Table 2 Rancimat induction period (hrs) at 110 C of argan oil prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO). Oil samples prepared from Ait Baha, Tidzi, and Tiout are indexed AB, TZ, and TT; respectively. Mean standard deviation of the values (five replicates) are presented.
110 C
MAOAB
MAOTZ
MAOTT
AAOAB
AAOTZ
AAOTT
GPAOAB
GPAOTZ
GPAOTT
24 0.5
27 0.5
31 1
18 0.5
16 0.5
14 0.5
14 0.5
16 0.5
16 0.5
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Table 3 Fatty acid distribution (initial and final) in argan oil samples prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) and stored for 2 years at 5 C, 25 C in clear or dark glass, and at 40 C. Values are expressed in g/100 g of total extracted fatty acids (1) and result from four replicates. Initial
Stored at 25 C in clear glass
Stored at 25 C in dark glass
Stored at 40 C
MAO Palmitic acid Stearic acid Oleic acid Linoleic acid
13 5 48 32
14 5 48 32
13 5 48 32
14 5 48 31
AAO Palmitic acid Stearic acid Oleic acid Linoleic acid
13 5 47 33
15 5 47 33
14 5 47 33
14 5 47 33
GPAO Palmitic acid Stearic acid Oleic acid Linoleic acid
14 6 48 31
15 6 47 30
14 6 48 31
15 6 48 30
prepared in the family circle. Therefore, it is highly likely that the short preservation time attributed to GPAO, compared to AAO, principally results from microbiologically-induced damage, rather than a chemically-assisted process. The homogenous results observed within each group during this preliminary study evidenced that consideration of the geographic origin was unnecessary for our study. Incidentally, we also decided to select 110 C as the optimum temperature to evaluate the oxidative stability of our oil samples since at this specific temperature afforded well reproducible results as already observed with olive oil (Mateos, Uceda, Aguilera, Escuderos, & Beltran Maza, 2006). 3.3. Preservation of argan oil, a two-year study Initial fatty acid, sterol and tocopherol composition was carried out prior the beginning of our study. Values are listed Tables 3e5. 3.3.1. Acid value analysis Acid value of MAO, AAO, and GPAO stored at 5 C did not significantly changed over two years. Initial acid values were 0.2 for
Table 4 Sterol composition (initial and final) in argan oil samples prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) and stored for 2 years at 5 C, 25 C in clear or dark glass, and at 40 C. Values are expressed in g/100 g of total sterols (2) and result from four replicates.
Table 5 Tocopherol composition (initial and final) in argan oil samples prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) and stored for 2 years at 5 C, 25 C in clear or dark glass, and at 40 C. Results are expressed in mg/kg and come from three replicates. Stored at 25 C in clear glass
Initial
Stored at 25 C in dark glass
Stored at 40 C
MAO Total a-tocopherol b-tocopherol g-tocopherol d-tocopherol
675 59 6 531 51
25 8 2 25 8
564 33 2 479 33
25 7 1 25 7
601 41.5 2 503 38.5
25 8 1 25 7
589 36.5 2 495 36
25 7 1 25 7
AAO Total a-tocopherol b-tocopherol g-tocopherol d-tocopherol
766 72 7 585 82
25 10 2 25 12
545 20 2 471 34
25 8 1 25 10
599 35 2 491 47
25 10 1 25 8
528 27 2 445 42
25 10 1 25 8
GPAO Total a-tocopherol b-tocopherol g-tocopherol d-tocopherol
660 70 5 531 39
25 10 2 25 7
462 35 2 386 20
25 10 1 25 6
559 43 3 467 31
25 10 1 25 8
518 40 2 437 25
25 10 1 25 7
MAO and AAO, and 0.9 for GPAO. After two years at 5 C, acid value was 0.2, 0.4, and 1 for MAO, AAO, and GPAO; respectively. Acid value of MAO also remained remarkably stable over two years independently on the storage temperature and glass color (Fig. 1). After two years at 25 C, acid value of MAO was 0.3. It was 0.4 after two years of storage at 40 C. Acid value of AAO stored in dark bottles at 25 C increased only very slightly (average 0.02 acid value unit/month) during the first 17 months of storage to reach the value of 0.5. After 17 months of storage, acid value increased 5-fold faster reflecting accelerated triacylglycerol degradation (Fig. 1). After 21 months at 25 C in colored glass bottles, acid value of AAO reached the 0.8 limit, loosing its extra virgin label (Service de normalisation industrielle, 2003). When AAO was stored unprotected from sunlight at 25 C, increase in acid value began two months sooner and the 0.8 limit was reached after 19 months (Fig. 1). However, after two years, final acid value of AAO stored in clear or dark bottles was similar: 1.2 0.2. Stored at 40 C, AAO lost its extra virgin label after 18 months. Initial acid value of GPAO was much higher than that of MAO and AAO and already above the 0.8 limit (Table 1). When stored at 25 C in clear glass bottles, acid value of GPAO significantly and continuously increased (Fig. 1) whereas that of samples stored at
Initial Stored at 25 C Stored at 25 C Stored in clear glass in dark glass at 40 C MAO Schottenol 46 Spinasterol 40 D-7-avenasterol 5.5 Stigmasta-8,22-dien-3b-ol 5
46.5 40 4 3.5
46.5 39 4 3.5
45 37 4 3
AAO Schottenol 44 Spinasterol 42 D-7-avenasterol 4 Stigmasta-8,22-dien-3b-ol 3.5
44 42 4 3
44 41 3.5 3
44 39 3 3
GPAO Schottenol 44 Spinasterol 43 D-7-avenasterol 6 Stigmasta-8,22-dien-3b-ol 4
44 42 3 3
44 41 4 4
44 40 3 3
Fig. 1. Acid value of argan oil as a function of time (months). Oil samples were either sunlight-protected and stored at 25 C (black symbols), exposed to sunlight and stored at 25 C (white symbols), or sunlight-protected and stored at 40 C (grey symbols). Oil was prepared from fruits either peeled by goats (rhombs), mechanically peeled (triangles), or manually peeled (squares). Mean standard deviation of the values (three replicates) are presented.
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Fig. 2. Peroxide value and K232 (bold line) of mechanically extracted argan oil (MAO) as a function of time (months). Samples were either protected from sunlight and stored at 25 C (rhombs), exposed to sunlight and stored at 25 C (triangles), or protected from sunlight and stored at 40 C (squares). Mean standard deviation of the values (three replicates) are presented.
the same temperature in dark bottles remained stable during 5 months (Fig. 1). At 40 C, GPAO acid value increased continuously, but less rapidly than the acid value of AAO at the same temperature (Fig. 1). Interestingly, the 2-year acid value profiles of MAO, AAO, and GPAO stored at 40 C or at 25 C and unprotected from sunlight were almost similar (Fig. 1). This result suggests that between 25 and 40 C, light is much more important than temperature to induce triacylglyceride oxidation in argan oil. 3.3.2. Peroxide value analysis Peroxides are the primary oxidation products that lead to rancidity. Therefore, their formation dramatically impacts oil shelf life and consumer acceptance. High temperature and light are two well-known factors generally promoting peroxide formation. In argan oil, the respective impact of these two factors is presently unknown. Initial peroxide value of MAO, AAO, and GPAO was found to be below 2 meq of O2/kg oil, well below the maximum peroxide value of 15 meq O2/kg oil defined for the extra virgin argan oil label (Service de normalisation industrielle, 2003). For MAO, AAO, as well as GPAO, storage at 5 C for two years led only to a very slight increase of the peroxide value (data not shown); the highest peroxide value of 3 meq O2/kg oil was observed for AAO. Peroxide value of MAO, AAO, and GPAO stored at 25 C or 40 C behaved differently. When stored at 25 C in dark or clear glass bottles, MAO peroxide value remained below the 15 meq O2/kg oil limit for two years. Accurate examination of the changes indicated that MAO peroxide value increased permanently over two years to reach the almost similar maximum values of 10.7 meq O2/kg oil for MAO protected from sunlight and 12 meq O2/kg oil for MAO exposed to sunlight (Fig. 2). However, the increase rate seems to be
light-dependent since the oxidation kinetic observed between light protected and unprotected samples was different (Fig. 2). Argan oil natural antioxidants are also likely to influence this kinetic. Storage of MAO in an oven at 40 C for two years led to peroxide values permanently higher than that observed when stored at 25 C. The final peroxide value was 16.3 meq O2/kg oil, whereas the limit of 15 meq O2/kg oil barrier was crossed after 21 months. Interestingly, the profiles of the peroxide values at 40 C and 25 C in dark bottles were similar. This likely means that a process identical, but amplified at 40 C, occurs in MAO protected from sunlight at 25 C or 40 C, and confirms that degradation and oxidative processes occurring in MAO are greatly accelerated under sunlight. AAO stored at 25 C in clear glass bottles crossed the 15 meq O2/ kg oil limit after 13 months. At the same temperature, 19 months were necessary when AAO was protected from sunlight. When AAO was stored at 25 C and exposed to sunlight, the peroxide value increased quite consistently over two years to reach 25.8 meq O2/kg oil after two years (Fig. 3). Sunlight protection led to lower peroxide values (14.4 meq O2/kg oil, and 18.3 meq O2/kg when stored at 25 C and 40 C; respectively) that were similar to that observed for MAO for corresponding storage conditions. However those values should be carefully handled since peroxide value underwent large fluctuations over two years. Such phenomenon was not observed for MAO possibly suggesting the occurrence for AAO of multiple secondary oxidation processes that did not occur in MAO and hence that could be related to the different content in minor components. With regards to the peroxide value, GPAO satisfied the virgin label requirements for 13 and 15 months when stored at 25 C in clear or dark glass bottles; respectively. When GPAO was stored at 25 C in clear glass bottles, peroxide value was at its highest
Fig. 3. Peroxide value and K232 (bold line) of artisanally extracted argan oil (AAO) as a function of time (months). Samples were either protected from sunlight and stored at 25 C (rhombs), exposed to sunlight and stored at 25 C (triangles), or protected from sunlight and stored at 40 C (squares). Mean standard deviation of the values (three replicates) are presented.
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Fig. 4. Peroxide value and K232 (bold line) of argan oil prepared from goat-peeled fruit (GPAO) as a function of time (months). Samples were either protected from sunlight and stored at 25 C (rhombs), exposed to sunlight and stored at 25 C (triangles), or protected from sunlight and stored at 40 C (squares). Mean standard deviation of the values (three replicates) are presented.
(19.8 meq O2/kg oil) after 15 months. Then, it decreased to reach the low value of 6.9 meq O2/kg oil after two years. This phenomenon was amplified at 40 C (Fig. 4). At 25 C and protected from sunlight, AAO and GPAO peroxide values behaved globally similarly. Therefore, as already observed from the acid value study, over two years, light also appears as the major parameter promoting hydroperoxide formation in all types of argan oil, elevated temperature favoring only secondary oxidation product formation. 3.3.3. K232 analysis Primary oxidation product formation can also be monitored by measuring specific extinction at 232 nm (K232). High quality argan oil should present a K232 lower than 2.5 (Service de normalisation industrielle, 2003). During two years of storage at 5 C, K232 of MAO, AAO and GPAO remained practically constant (initial value 1.06, 1.24, and 1.28 vs final value 1.27, 1.29, and 1.52 for MAO, AAO, and GPAO; respectively) as expected from the results of the peroxide value study. When MAO was stored at higher temperature, K232 was observed between 1.6 and 2 after 2 years. When stored at 25 C in clear or dark glass bottles, K232 and peroxide value evolved in a similar way (Fig. 2), suggesting the low incidence of secondary oxidative product formation. During storage at 40 C, although peroxide value increased swiftly between months 11 and 17, K232 absorption remained quite stable during this period. This strongly suggests the occurrence at 40 C of multiple, complex, and not fully identified oxidative processes for which the involvement of phospholipids can be eliminated since a similar behavior was also observed for AAO and GPAO (Figs. 3 and 4). Whereas MAO had K232 between 1.6 and 2 after two years, AAO K232 was between 1.8 and 3 after the same period of time. AAO K232 crossed the 2.5 barrier after 11 and 17 months of storage at 25 C in dark and clear glass bottles; respectively (Fig. 3). Surprisingly, during storage 40 C, K232 of AAO never reached the 2.5 value even though observed peroxide value reflected the occurrence of intense oxidative processes. Consequently, in that case, a direct correlation between peroxide value and K232 was uneasy to establish, likely due to multiple and concomitant oxidation processes favored by temperature. Finally, for GPAO, K232 after two years was between 1.6 and 2.4. K232 absorption remained surprisingly stable when GPAO was stored at 25 C in dark bottles. When stored at 25 C in clear bottles, a good correlation was observed between K232 and peroxide value, both indexes decreasing after 15 or 17 months (Fig. 4). During storage at 40 C, K232 and peroxide value increased simultaneously until month 17 but K232 remained stable although peroxide value dramatically plummeted after this month.
3.3.4. K270 study Carbonyl (aldehyde and ketone) compounds are the most abundant secondary oxidation products formed in edible oils. Their formation is known to be accelerated by elevated temperature and metal traces (Choe & Min, 2006). UV absorption at l 270 nm (K270) is one of the markers used to follow secondary oxidation formation. Moroccan regulation has set the maximum value for K270 at 0.35 (Service de normalisation industrielle, 2003). Overall, K270 values did not significantly changed over the 2 years. Initial values are given in Table 1. That argan oil samples stored at 5 C over 2 years displayed stable K270 was not surprising. That this trend also occurred for oil samples stored at higher temperature was unexpected. Final K270 values for MAO were 0.24, and 0.31 when samples were stored at 25 C, and 40 C, respectively. For GPAO, final values were in the same range for the three storage conditions. Only K270 of AAO stored at 40 C crossed the limit value of 0.35 after 17 months to end up at 0.39 0.05. Independently of the sunlight protection, the final value of AAO stored at 25 C was 0.21 0.05. In summary, K232 and peroxide value depict the formation of primary oxidation products. The apparent sample-dependent correlation observed between K232 and peroxide value supports the idea of different ratio of hydroperoxides depending on the type of argan oil. Decomposition of these hydroperoxides into secondary oxidation products can be monitored by K270 examination. Our results show that hydroperoxides formed in the three types of
Table 6 Rancimat induction period (hrs) at 110 C of argan oil prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) and stored at 5 C, 25 C (clear or dark glass bottles), and 40 C for up to 2 years. Mean standard deviation of the values (five replicates) are presented. 6 Months 12 Months 18 Months
Final
MAO Stored Stored Stored Stored
at at at at
5 C 25 C in clear glass 25 C in dark glass 40 C
32 28 29 28
1 1 1 1
30 28 28 27
1 1 1 1
31 26 27 26
1 1 1 1
30 25 27 24
1 1 1 1
AAO Stored Stored Stored Stored
at at at at
5 C 25 C in clear glass 25 C in dark glass 40 C
13 11 13 10
0.5 0.5 0.5 0.5
12 11 12 10
0.5 0.5 0.5 0.5
13 10 11 10
0.5 0.5 0.5 0.5
14 9 10 8
0.5 0.5 0.5 0.5
GPAO Stored Stored Stored Stored
at at at at
5 C 25 C in clear glass 25 C in dark glass 40 C
15 13 15 12
0.5 0.5 0.5 0.5
14 12 13 12
0.5 0.5 0.5 0.5
15 11 12 11
0.5 0.5 0.5 0.5
15 10 12 9
0.5 0.5 0.5 0.5
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Table 7 Saponification value, iodine index, and b-carotene level (initial and final) of argan oil samples prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) stored for 2 years at 5 C, 25 C in clear or dark glass, and at 40 C. Mean standard deviation of the values (five replicates) are presented. Stored at 5 C
Stored at 25 C in clear glass
Stored at 25 C in dark glass
Stored at 40 C
Saponification value (mg of KOH/g of oil) MAO 189.5 0.5 AAO 192.6 0.5 GPAO 190.6 0.4
189.7 0.2 192.7 0.5 192.5 0.5
190.8 0.5 193 0.5 191.7 0.6
190.6 0.4 193.2 0.2 191 0.5
194 0.1 193.9 0.5 193.4 0.6
Iodine index (g of I2/100 g of oil) MAO 97.7 0.1 AAO 102.4 0.5 GPAO 96.8 0.5
96.9 0.5 101.2 0.5 95.7 0.5
96.7 0.5 99.4 0.5 94.5 0.5
96.7 0.5 99.4 0.6 96.4 0.5
95.9 0.5 98.3 0.4 95.6 0.5
18.8 0.5 17.9 0.5 15.4 0.5
17 0.5 7.1 0.5 6.6 4
17.4 0.5 10.1 0.5 10.2 5
17 0.5 11.3 0.5 15.4 5
Initial
b-Carotene level (mg/kg) MAO AAO GPAO
20.7 0.5 18 0.5 17.5 0.5
argan oil decompose to unsaturated secondary oxidation products, and that MAO presents the slowest decomposition rate. In GPAO, the oxidative profile is more complex. Keeping in mind that MAO presents a highly homogeneous chemical composition, likely induced by its highly homogeneous geographical origin, these observations are of the utmost importance, considering the negative influence of secondary oxidation products on oil taste and smell, from an organoleptic standpoint. 3.3.5. Rancimat study Then we investigated the oil oxidative stability by measuring every 6 months the rancimat induction period at 110 C of MAO, AAO and GPAO stored in our evaluated conditions. Results are reported Table 6. When oil samples were stored at 5 C, rancimat induction period did not significantly vary over two years. Amazingly, at storage temperatures above 5 C, rancimat induction period of each type of oil decreased during the first 6 months then remained almost unchanged during the last eighteen months. MAO displayed by far the longest induction period confirming the good preservation properties of this type of oil. Rancimat induction time of oil samples stored at 25 C and exposed to sunlight was found to be slightly but significantly shorter than that of oil samples stored at the same temperature but protected from sunlight. This result is consistent with our previous observations that indicate the occurrence of a slower oxidative process in argan oil samples protected from sunlight. Storage of argan oil at 40 C for 2 years led to a reduction of the rancimat induction period almost similar to that observed for argan oil stored at 25 C and exposed from sunlight. 3.3.6. Miscealenous analyses Finally, we also decided to analyze some of the physicochemical parameters of our oil samples after two years in order to possibly detect variations affecting its pharmacologically essential components. Because most of argan oil therapeutic properties are linked to its high unsaturated fatty acid content, we determined several parameters including the iodine index, saponification value, and fatty acid composition of every oil samples after 2 years (Tables 3e5, 7). Concerning the saponification value, the largest variation was observed for samples stored at 40 C but the saponification value calculated after 2 years was still satisfying the official norm (Service de normalisation industrielle, 2003). Over two years, iodine index underwent a minor reduction due to primary oxidation but, for all samples it was consistently found between 91 and 110 as required by the official norm (Service de normalisation industrielle, 2003). Additionally, we also analyzed the b-carotene content of our oil samples since b-carotene actively participates in
oleic-rich oil protection under autooxidative and photooxidative processes (Goulson & Warthesen, 1999). While b-carotene level in MAO remained stable over two years, a strong decrease was observed for AAO stored at 25 C in clear glass bottles likely due to intense oxidative reactions. Measurements for GPAO were inconclusive due to the color variation of the oil samples after two years. Concerning the fatty acid and sterol distribution in each oil samples, no significant changes were observed over two years. Results are listed Tables 3 and 4. Tocopherols possess antioxidative and anti free-radical properties. Therefore oil oxidative stability depends on changes occurring in tocopherol content during storage (Okogeri & Tasioula-Margari, 2002). Tocopherol high concentration in argan oil is not only essential for its preservation but also for its pharmacological activity (Khallouki et al., 2003). Storage of argan oil for two years at 25 C in clear glass bottles resulted in a dramatic decrease in tocopherol level for the three types of oil. Sunlight protection resulted in a reduced tocopherol lost that was nevertheless consequent for oil samples stored at 40 C. Individually considered, a-, b-, and d-tocopherol levels were almost divided by two after two years of storage in clear glass bottles or at 40 C. Only storage in dark-glass bottles allowed the preservation of a high g-tocopherol level (Table 5). 4. Conclusions Combined all together, our results designate light as the major factor involved in argan oil oxidation. After two years of storage at 25 C, MAO protected from sunlight displays several physicochemical properties and an oxidative induction period that remained similar to freshly prepared argan oil. MAO is the type of argan oil that is sold on the international market and a shelf life of two years can reasonably be recommended for such oil as long as it is protected from sunlight. Edible argan oil has a characteristic copper color that helps consumers to distinguish it rapidly from other oils. Therefore the use of colored glass bottles is unlikely to be easily accepted by a majority of consumers. Argan bottles are generally packed in cardboard box, such practice should be preserved since to help argan oil preservation. Acknowledgments This work was performed in the frame of “Projet arganier” and financially supported by “Agence de Développement Social” and EEC (#AR05A061P704). We thank Association Ibn Al Baytar, Lesieur-Cristal, cooperatives Targant (Ait Baha), Taitmatine (Tiout), and Tidzi cooperative for their support and assistance in this work.
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References American Oil Chemists’ Society. (1993). Determination of tocopherols and tocotrienols in vegetable oils and fats by HPLC. Uniform Methods Committee. AOCS Official Method Ce 8e89. Champaign, Il: AOCS. Cayuela, J. A., Rada, M., Pérez-Camino, M. C., Benaissa, M., Abdelaziz, E., & Guinda, A. (2008). Characterization of artisanally and semiautomatically extracted argan oils from Morocco. European Journal of Lipid Science Technology, 110, 1159e1166. Charrouf, Z., El Hamchi, H., Mallia, S., Licitra, G., & Guillaume, D. (2006). Influence of roasting and seed collection on argan oil odorant composition. Natural Product Communications, 1, 399e404. Charrouf, Z., & Guillaume, D. (1999). Ethnoeconomical, ethnomedical and phytochemical study of Argania spinosa (L.) Skeels. Journal of Ethnopharmacology, 67, 7e14. Charrouf, Z., & Guillaume, D. (2002). Secondary metabolites from Argania spinosa (L.) Skeels. Phytochemistry Reviews, 1, 345e354. Charrouf, Z., & Guillaume, D. (2008). Argan oil: occurrence, composition and impact on human health. European Journal of Lipid Science Technology, 110, 632e636. Charrouf, Z., & Guillaume, D. (2009). Sustainable development in Northern Africa: the argan forest case. Sustainability, 1, 1012e1022. Charrouf, Z., & Guillaume, D. (2010). Should the Amazigh diet (regular and moderate argan-oil consumption) have a beneficial impact on human health? Critical Review in Food Science and Nutrition, 50, 473e477. Charrouf, Z., Guillaume, D., & Driouich, A. (2002). The argan tree, an asset for Morocco. Biofutur, 220, 54e57, (in French). Chimi, H. (2005). Comparative preservation: argan oil and olive oil. Cahiers Agriculture, 14, 467e471. Chimi, H., Cillard, J., & Cillard, P. (1994). Autoxidation of argan oil (Argania spinosa L.) from Morocco. Sciences des Aliments, 14, 117e124. Choe, E., & Min, D. B. (2006). Mechanisms and factors for edible oil oxidation. Comprehensive Review in Food Science and Food Safety, 5, 169e186. Cinquinta, L., Esti, M., & La Notte, E. (1997). Evolution of phenolic compounds in virgin olive oil during storage. Journal of the American Oil Chemist’s Society, 74,1259e1264. Collier, A., & Lemaire, B. (1974). Carotenoids of argan oil. Cahiers Nutrition Diététique, 9, 300e301. Goulson, M. J., & Warthesen, J. J. (1999). Stability and antioxidant activity of beta carotene in conventional and high oleic canola oil. Journal of Food Science, 64, 996e999. Guillaume, D., & Charrouf, Z. (2005). Saponines et métabolites secondaires de l’arganier (Argania spinosa): état des connaissances. Cahiers Agriculture, 15, 509e516. Hilali, M., Charrouf, Z., El Aziz Soulhi, A., Hachimi, L., & Guillaume, D. (2005). Influence of origin and extraction method on argan oil physico-chemical
characteristics and composition. Journal of Agricultural and Food Chemistry, 53, 2081e2087. Judde, A., Villeneuve, P., Rossignol-Castera, A., & Le Guillou, A. (2003). Antioxidant effect of soy lecithins on vegetable oil stability and their synergism with tocopherols. Journal of the American Oil Chemist’s Society, 80, 1209e1215. Khallouki, F., Younos, C., Soulimani, R., Oster, T., Charrouf, Z., Spiegelhalder, B., et al. (2003). Consumption of argan oil (Morocco) with its unique profile of fatty acids, tocopherols, squalene, sterols and phenolic compounds should confer valuable cancer chemopreventive effects. European Journal of Cancer Prevention, 12, 67e75. Koga, T., & Terao, J. (1995). Phospholipids increase radical-scavenging activity of vitamin E in a bulk oil model system. Journal of Agricultural and Food Chemistry, 43, 1450e1454. Koprivnjak, O., Skevin, D., Valic, S., Majetic, V., Petricevic, S., & Ljubenkov, I. (2008). The antioxidant capacity and oxidative stability of virgin olive oil enriched with phospholipids. Food Chemistry, 111, 121e126. Marfil, R., Cabrera-Vique, C., Giménez, R., Bouzas, P. R., Martínez, O., & Sánchez, J. A. (2008). Metal content and physicochemical parameters used as quality criteria in virgin argan oil: influence of the extraction method. Journal of Agricultural and Food Chemistry, 56, 7279e7284. Mateos, R., Uceda, M., Aguilera, M. A., Escuderos, M. E., & Beltran Maza, G. (2006). Relationship of rancimat method values at varying temperatures for virgin olive oils. European Food Research and Technology, 223, 246e252. Matthäus, B., Guillaume, D., Gharby, S., Haddad, A., Harhar, H., & Charrouf, Z. (2010). Effect of processing on the quality of edible argan oil. Food Chemistry, 120, 426e432. Morton, J. F., & Voss, G. L. (1987). The argan tree (Argania sideroxylon, Sapotaceae), a desert source of edible oil. Economic Botany, 41, 221e233. Okogeri, O., & Tasioula-Margari, M. (2002). Changes occurring in phenolic compounds and a-tocopherol of virgin olive oil during storage. Journal of Agricultural and Food Chemistry, 50, 1077e1080. Paquot, C., & Hautfenne, A. (1987). IUPAC, Standard methods for the analysis of oils, fats and derivatives (7th ed.). Oxford, England: Blackwell Scientific Publications. IUPAC, and Standard methods for the analysis of oils, fats and derivatives (Suppl. 1, 7th ed.) (1992). Oxford, England: Pergamon Press. Rahmani, M. (2005). Composition chimique de l’huile d’argane vierge. Cahiers Agriculture, 14, 461e465. Service de normalisation industrielle (Snima). (2003). Corps gras d’origine animale et végétale- Huiles d’argane. Spécifications, Norme Marocaine NM 08.5.090. Rabat (Morocco): SNIMA. Tatum, V., & Chow, C. K. (1992). Effects of processing and storage on fatty acids in edible oils. In C. K. Chow (Ed.), Fatty acids in foods and their health implications (pp. 411e425). New York: Marcel Dekker.
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Oxidative stability of edible argan oil: A two-year study Saïd Gharby a, b, Hicham Harhar a, Dominique Guillaume c, *, Aziza Haddad b, Bertrand Matthäus d, Zoubida Charrouf a a
Laboratoire de Chimie des Plantes et Synthèse Organique, Département de Chimie, Faculté des Sciences, Université MohammedV-Agdal, BP1014, Rabat, Morocco Laboratoire Contrôle Qualité, Lesieur-Cristal, 1 rue Caporal Corbi, 20300 Roches Noires-Casablanca, Morocco c CNRS-UMR6229, Université de Reims-Champagne Ardenne, 51 rue Cognacq Jay, 51100 Reims, France d Max Rubner-Institute, Federal Research Institute for Nutrition and Food, Department for Lipid Research, Piusallee 68/76, 48147 Münster, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 March 2010 Received in revised form 31 May 2010 Accepted 2 July 2010
The present study investigated the oxidative stability of the threemarketed types of edible argan oil. Edible argan oil is prepared by pressing the slightly roasted kernels of peeled argan fruit. High quality edible argan oil is exclusively prepared using mechanical presses. However, hand-extracted argan oil is still artisanally produced and can be found in local markets. In this latter case, goat-peeled fruit is still sometimes introduced in the oil production chain even though the resulting oil is notoriously of unsatisfactory quality. The oxidative stability of press-extracted, hand-extracted, and goat-peeled fruit derived argan oil was analyzed using as physicochemical metrics: fatty acid composition, b-carotene level, phosphorus level, tocopherol level, iodine index, saponification, peroxide and acid values, specific extinction, and Rancimat induction time. The variations of these parameters were evaluated over a period of 2 years at 5 C, 25 C (protected or exposed to sunlight), or 40 C. After this period of time, mechanically extracted argan oil still presents an excellent physicochemical profile. Domestic and traditionally prepared argan oil presents much less satisfactory properties after the same period. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Argania spinosa Edible argan oil Long-term oil preservation Long-term oil quality
1. Introduction The main consequence of the drastic improvements recently brought to argan oil preparative process (Charrouf, Guillaume, & Driouich, 2002) was its entry as a major actor in the edible-andexpensive-oil closed circle. Indeed, argan oil that was almost unknown out of the limits of the argan forest twenty years ago is now sold in virtually every gourmet-stores around the world. Edible argan oil is the basis of the Amazigh diet (Charrouf & Guillaume, 2010). It is prepared from the roasted kernels of the fruit of the argan tree (Argania spinosa (L.) Skeels) that is exclusively endemic in Southern Morocco (Morton & Voss, 1987). Recent attempts to sustainably develop this region (Charrouf & Guillaume, 2009) threatened by desertification have been consecutive to its designation as a Biosphere Reserve by UNESCO in 1998 and to our early intensive chemical work principally carried out on argan tree metabolites (Charrouf & Guillaume, 2002; Guillaume & Charrouf, 2005) and its fruit-derived oil (Charrouf & Guillaume, 1999). Some of these efforts have been fruitful and have led to the implantation in the argan forest of several argan oil-producing
* Corresponding author. Tel.: þ33 326918473; fax: þ33 326918029. E-mail address: [email protected] (D. Guillaume). 0023-6438/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2010.07.003
woman cooperatives where high quality argan oil is now prepared using a strictly controlled process. Particularly, new rules include 1) the banishment of goat-peeled fruit, and therefore the necessary use of scratching machines to peel the argan fruit, and 2) the use of screw-presses to extract the oil in place of water-requiring hand malaxing of argan dough. Consequently, low-grade argan oil has been gradually replaced by high quality argan oil. The combination of high levels of unsaturated fatty acids and antioxidants, and its unique taste and pharmacological properties (Charrouf & Guillaume, 2008), have ultimately boosted high quality argan oil market share. If the negative influence of hand-malaxing, uncontrolled kernelroasting time, or “goat-peeling” on argan oil quality is now particularly well documented (Charrouf, El Hamchi, Mallia, Licitra, & Guillaume, 2006; Hilali, Charrouf, El Aziz Soulhi, Hachimi, & Guillaume, 2005), the real improvement in terms of oil preservation time has not been precisely studied and quantified, yet. To fill this gap, we decided to evaluate over a two year period the oxidative stability of argan oil prepared 1) traditionally (hand malaxed), 2) mechanically (screw-pressed), and 3) using goatpeeled fruit (hand malaxed and animal-peeled). Such a study is highly desirable to determine an accurate shelf life for each type of oil on the domestic or international market. During our study, oil samples were kept at 5 C, 25 C, or 40 C. Because oil samples
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refrigerated at 5 C or heated at 40 C were necessarily sunlight protected (fridge or oven; respectively), and since light is wellknown to possibly influence edible oil oxidation (Cinquinta, Esti, & La Notte, 1997), we also decided to evaluate the influence of light on our oil samples kept at 25 C by use of clear glass or dark-glass bottles. Therefore, twelve oil samples were periodically analyzed over a two year period. To be able to eventually link possible characteristic variations to a preparative process, all our studied oil samples were prepared in the same cooperative. Consequently, to ascertain the general character of our results, before the beginning of our study, we first demonstrated the lack of influence of the geographic origin on argan oil initial physicochemical parameters by comparing the oxidative stability of argan oil produced at three different locations of the argan forest. 2. Materials and methods 2.1. Chemicals and materials All the reagents were of analytical or HPLC grade. Isooctane and isopropanol used as HPLC mobile phase and cyclohexane used for extinction coefficient determination were purchased from Professional Labo (Casablanca, Morocco). Clear- and brown-glass bottles were purchased from Cfimu Sarl (Casablanca, Morocco). 2.2. Sample collection Argan oil samples analyzed to determine the initial oxidative parameters were prepared in 2006 in the woman cooperatives of Ait Baha (Chtouka-Ait Baha county, Morocco), Tidzi (Essaouira county, Morocco), and Tiout (Taroudant county, Morocco) following our previously reported protocol (Hilali et al., 2005). For the twoyear study, argan oil samples were those prepared in the woman cooperative of Tiout. 2.3. Sample distribution For the determination of the initial physicochemical parameters of argan oil, three types of oil were prepared: artisanally extracted argan oil (AAO), mechanically extracted argan oil (MAO), and traditional artisanal argan oil obtained from goat-peeled fruit (GPAO). Oil samples prepared from Ait Baha, Tidzi, and Tiout are indexed AB, TZ, and TT; respectively. Time-dependent oxidative stability was studied by comparing the physicochemical properties of twelve samples. Ten liters of AAO, MAO, and GPAO were prepared. Each oil type was distributed in 360 60-mL-glass bottles: 270 clear- and 90 brown-glass bottles. The remaining oil was used to determine initial values. For a given oil type, 30 clear-glass bottles were stored at 5 C, 25 C, and 40 C. Additionally, 30 brown-glass bottles were stored at 25 C. Headspace volume (bottleneck volume) for each bottle was 3.5 (0.5) mL. 2.4. Analytical methods Samples stored at 5 C were analyzed after 5, 11, 17, and 23 months of storage. Samples stored at 25 C or 40 C were analyzed after 1 month of storage then every 2 months until month 23. Acid value, peroxide value, saponification value, iodine index, and UV-light absorption (K270 and K232) were determined as previously described (Hilali et al., 2005). For the fatty acid composition determination, the methyl esters were analyzed on a CP-Wax 52CB column (30 m 0.25 mm i.d.) using helium (flow rate 1 mL/mn) as a carrier gas. Initial oven temperature
was set at 170 C; injector temperature 200 C; detector temperature 230 C. Injected quantity was 1 mL for each analysis. The oxidative stability of each sample was determined as the induction period (IP, hours) recorded by a Rancimat 743 (Metrohm) apparatus using 3 g of oil sample with an air flow of 20 L/h. To identify the initial oxidative parameters, oxidative stability was successively determined at 90 C, 100 C, 110 C, 120 C, 130 C, and 140 C. For the two-year study, IP was determined at 110 C. Sterol composition was determined after trimethylsilylation of the crude sterol fraction. Trimethylsilylated derivatives were analyzed by gas chromatography using a Varian 3800 instrument equipped with a VF-1ms column (30 m 0.25 mm i.d.) using helium (flow rate 1.6 mL/mn) as carrier gas. Column temperature was isothermal at 270 C, injector and detector temperature was 300 C. Injected quantity was 1 mL for each analysis. Individual tocopherol content was determined on the basis of the AOCS Official method Ce 8-89 (American Oil Chemists’ Society,1993). Tocopherols were analyzed by HPLC using Shimadzu instruments equipped with a C18-Varian column (25 cm 4 mm). Detection was performed using a fluorescence detector (excitation wavelength 290 nm, detection wavelength 330 nm). Eluent used was a 99:1 isooctane/isopropanol (V/V) mixture, flow rate 1.2 mL/mn. Phosphorus content was determined using the NF T60-227 recommendation (Paquot & Hautfenne, 1987). b-Carotene content was determined using a PFX-995 lovibond tintometer (cell length 10 mm). 2.5. Statistical analysis Values reported in tables and figures are the means SE of 3e5 replications. The significance level was set at P ¼ 0.05. Separation of means was performed by Turkey’s test at the 0.05 significance level. 3. Results and discussion Genuine edible argan oil is exclusively prepared in Morocco since argan trees are only endemic in this country. Three types of edible argan oil can be found on the market: “certified”, “artisanal”, and “family”. Those denominations reflect the oil preparative process. Certified argan oil is exclusively prepared in woman cooperatives by use of mechanical presses; it is sold on both domestic and international markets. Artisanal argan oil is prepared by decanting the liquid resulting from the prolonged mixture of argan dough and water; it is mainly sold on Morocco domestic market but can also be purchased on the Internet. Family argan oil is generally prepared under rudimentary conditions with the risk of bacteriologically unsafe water and use of goat-peeled fruit; it is used in the family circle but surplus of oil is sometimes sold on the local market. Each type of oil presents its own physicochemical (Hilali et al., 2005) and organoleptic profile (Matthäus et al., 2010). The aim of our study was to evaluate the influence of a prolonged storage on argan oil physicochemical properties and oxidative stability. Consumption of argan oil usually occurs within 18e24 months. Accordingly, we chose a maximum storage time of two years for our study. Variations in oil processing can influence the initial oxidation of edible oil (Tatum & Chow, 1992). Therefore, we decided to determine the oxidative properties of the three common types of edible argan oil: mechanically-extracted (MAO), artisanally-extracted (AAO), and handextracted using goat-peeled fruit (GPAO). Four different storage conditions: refrigerated at 5 C, 25 C light-unprotected, 25 C lightprotected, and 40 C (oven) were considered. Our study began with the careful determination of the initial parameters of our samples. Due to the large area covered by the argan forest, the question of the incorporation of the oil geographic origin as a to-be-considered parameter came out rapidly. Consequently, we first decided to carry
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Table 1 Physicochemical parameters of argan oil prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goatpeeled fruit (GPAO). Oil samples prepared from Ait Baha, Tidzi, and Tiout are indexed AB, TZ, and TT; respectively. Mean standard deviation of the values (five replicates) are presented. MAOTZ
MAOAB Acid Value (mg/g) Peroxide value (Meq/kg) Moisture (mg/100 mg) K232 K270 b-carotene (ppm) Phosphorus (mg/103 g) Phospholipid (mg/100 mg) SFAa (mg/100 mg) UFAa (mg/100 mg) a
0.3 0.7 0.07 1.44 0.25 20 42.8 0.3 19 80
0.05 0.1 0.01 0.06 0.05 0.5 0.2 0.7 1
0.3 1.2 0.06 1.18 0.2 20 61.5 0.2 17.7 82
MAOTT 0.05 0.1 0.01 0.07 0.05 0.5 0.8 0.5 0.5
0.3 0.6 0.05 1.02 0.18 21 80.2 0.25 19.3 80
AAOAB 0.02 0.1 0.01 0.06 0.05 0.5 0.8
0.3 0.5
0.6 1 0.08 1.28 0.19 11 7.8 0.02 19.1 81
AAOTZ 0.1 0.1 0.01 0.06 0.05 0.5 0.1
0.3 0.5
0.5 1 0.06 1.21 0.18 13 5.3 0.01 18.2 81
AAOTT 0.1 0.1 0.02 0.06 0.05 0.5 0.1
0.1 0.5
0.3 1 0.08 1.24 0.22 18 3.9 0.01 19.6 79
GPAOAB 0.1 0.2 0.01 0.06 0.05 0.5 0.1
0.1 0.5
0.3 1.1 0.06 1.29 0.21 15 3.6 0.01 20 78.5
0.1 0.1 0.01 0.07 0.05 0.3 0.2
0.4 0.5
GPAOTZ 1.1 1.3 0.25 1.49 0.18 16 9.1 0.03 17 82
0.1 0.1 0.01 0.06 0.05 0.5 0.1
0.2 0.5
GPAOTT 0.7 1.5 0.09 1.37 0.17 17.5 6.1 0.02 18 80.5
0.1 0.2 0.01 0.06 0.05 0.5 0.1
0.2 0.2
SFA: saturated fatty acids, UFA: unsaturated fatty acids.
out an oxidative stability study/physicochemical analysis of argan oil samples coming from the three main locations, in terms of argan oil production, of the argan forest. 3.1. Determination of the initial physicochemical parameters of the oil samples Some physicochemical parameters of freshly prepared argan oil have already been reported to be either poorly dependent or independent on the nut harvest location (Cayuela et al., 2008; Charrouf et al., 2006). Because 1) short term autoxidation of argan oil has been only partially studied (Chimi, 2005; Chimi, Cillard, & Cillard, 1994), 2) the aspect of preservation has never been investigated in previous studies, 3) equipments used to obtain mechanically extracted argan oil are regularly upgraded, and 4) minute traces of metals can modify oil quality (Marfil et al., 2008), we decided to examine the physicochemical parameters of argan oil samples coming from Ait Baha (AB), Tidzi (TZ), and Tiout (TT), the three largest argan oil woman cooperatives in the argan forest. Table 1 lists the results of the physicochemical parameters analyzed. Satisfactorily, all fresh argan oil samples displayed the physicochemical properties necessary to access the edible grade as defined by the recommendations of the official argan oil norm (Service de normalisation industrielle, 2003). Nevertheless, the high acid value of GPAOTZ (>0.8%) was incompatible with an “extra virgin” label, even though it was acceptable for a “pure virgin” grade (Service de normalisation industrielle, 2003). Comparison between the determined parameters indicated that MAO consistently contained a significantly higher level of phospholipid/phosphorus than AAO and GPAO. It is likely that the development of heat at the press head during mechanical extraction results in a transfer of phospholipids into the oil and hence to a high amount of phospholipids in MAO. The low amount of phospholipids in AAO and GPAO results from a poor phospholipid extraction at room temperature. Phospholipids can trigger technical problems during oil degumming or refining. In our case, such a consideration is only of limited importance since, oppositely to cosmetic argan oil, edible (extra) virgin argan oil is not refined. More importantly, phospholipids can act as antioxidants or prooxidants (Choe & Min, 2006; Koprivnjak et al., 2008) depending on their concentration and the
presence of metal ions (Choe & Min, 2006) or tocopherols (Judde, Villeneuve, Rossignol-Castera, & Le Guillou, 2003; Koga & Terao, 1995). Since argan oil is notoriously rich in tocopherols, the high content in phospholipids in MAO, compared to AAO and GPAO should merit further attention. Other studied parameters ((un) saturated fatty acid, b-carotene level, UV absorption) were remarkably constant, any of them significantly varying as a function of the oil geographic origin. 3.2. Determination of the initial oxidative stability of the oil samples Conversely to argan oil physicochemical parameters, its oxidative stability has never been studied as a function of the oil geographic origin. To get a complete picture of argan oil oxidative stability, we decided to determine the induction period by Rancimat test at 90, 100, 110, 120, 130, and 140 C of our oil samples prepared in the three different locations. Results obtained at 110 C are presented Table 2. Interestingly, homogenous Rancimat induction periods were observed within each group. For every given temperature, MAO consistently and independently of the geographic origin displayed much longer rancimat induction periods than AAO or GPAO. Oxidative stability of argan oil has been attributed to its high content in tocopherols (Rahmani, 2005), and carotenes (Collier & Lemaire, 1974). Since these families of components are in a similar quantity in the three types of oil (Hilali et al., 2005), our results show that in fresh argan oil, phospholipids do not act as prooxidants, and presumably act as antioxidants, reinforcing the strong preservation activity of tocopherols. Notably large induction period differences between the MAO group and the AAO and GPAO groups were observed between 90 and 110 C. At 90 C, MAO average rancimat induction period was found to be 110 6 h, whereas it was only 76 6 h, and 70 2 h for AAO, and GPAO, respectively. At 110 C, the difference between the mean rancimat induction period of MAO and AAO or GPAO was 11 2 h. Unexpectedly, AAO and GPAO displayed similar rancimat induction periods even though GPAO preservation time is commonly said to be low. This apparent contradiction can be explained since in our study AAO and GPAO samples were prepared using bacteriologically safe water, a parameter never controlled when argan oil is
Table 2 Rancimat induction period (hrs) at 110 C of argan oil prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO). Oil samples prepared from Ait Baha, Tidzi, and Tiout are indexed AB, TZ, and TT; respectively. Mean standard deviation of the values (five replicates) are presented.
110 C
MAOAB
MAOTZ
MAOTT
AAOAB
AAOTZ
AAOTT
GPAOAB
GPAOTZ
GPAOTT
24 0.5
27 0.5
31 1
18 0.5
16 0.5
14 0.5
14 0.5
16 0.5
16 0.5
4
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Table 3 Fatty acid distribution (initial and final) in argan oil samples prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) and stored for 2 years at 5 C, 25 C in clear or dark glass, and at 40 C. Values are expressed in g/100 g of total extracted fatty acids (1) and result from four replicates. Initial
Stored at 25 C in clear glass
Stored at 25 C in dark glass
Stored at 40 C
MAO Palmitic acid Stearic acid Oleic acid Linoleic acid
13 5 48 32
14 5 48 32
13 5 48 32
14 5 48 31
AAO Palmitic acid Stearic acid Oleic acid Linoleic acid
13 5 47 33
15 5 47 33
14 5 47 33
14 5 47 33
GPAO Palmitic acid Stearic acid Oleic acid Linoleic acid
14 6 48 31
15 6 47 30
14 6 48 31
15 6 48 30
prepared in the family circle. Therefore, it is highly likely that the short preservation time attributed to GPAO, compared to AAO, principally results from microbiologically-induced damage, rather than a chemically-assisted process. The homogenous results observed within each group during this preliminary study evidenced that consideration of the geographic origin was unnecessary for our study. Incidentally, we also decided to select 110 C as the optimum temperature to evaluate the oxidative stability of our oil samples since at this specific temperature afforded well reproducible results as already observed with olive oil (Mateos, Uceda, Aguilera, Escuderos, & Beltran Maza, 2006). 3.3. Preservation of argan oil, a two-year study Initial fatty acid, sterol and tocopherol composition was carried out prior the beginning of our study. Values are listed Tables 3e5. 3.3.1. Acid value analysis Acid value of MAO, AAO, and GPAO stored at 5 C did not significantly changed over two years. Initial acid values were 0.2 for
Table 4 Sterol composition (initial and final) in argan oil samples prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) and stored for 2 years at 5 C, 25 C in clear or dark glass, and at 40 C. Values are expressed in g/100 g of total sterols (2) and result from four replicates.
Table 5 Tocopherol composition (initial and final) in argan oil samples prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) and stored for 2 years at 5 C, 25 C in clear or dark glass, and at 40 C. Results are expressed in mg/kg and come from three replicates. Stored at 25 C in clear glass
Initial
Stored at 25 C in dark glass
Stored at 40 C
MAO Total a-tocopherol b-tocopherol g-tocopherol d-tocopherol
675 59 6 531 51
25 8 2 25 8
564 33 2 479 33
25 7 1 25 7
601 41.5 2 503 38.5
25 8 1 25 7
589 36.5 2 495 36
25 7 1 25 7
AAO Total a-tocopherol b-tocopherol g-tocopherol d-tocopherol
766 72 7 585 82
25 10 2 25 12
545 20 2 471 34
25 8 1 25 10
599 35 2 491 47
25 10 1 25 8
528 27 2 445 42
25 10 1 25 8
GPAO Total a-tocopherol b-tocopherol g-tocopherol d-tocopherol
660 70 5 531 39
25 10 2 25 7
462 35 2 386 20
25 10 1 25 6
559 43 3 467 31
25 10 1 25 8
518 40 2 437 25
25 10 1 25 7
MAO and AAO, and 0.9 for GPAO. After two years at 5 C, acid value was 0.2, 0.4, and 1 for MAO, AAO, and GPAO; respectively. Acid value of MAO also remained remarkably stable over two years independently on the storage temperature and glass color (Fig. 1). After two years at 25 C, acid value of MAO was 0.3. It was 0.4 after two years of storage at 40 C. Acid value of AAO stored in dark bottles at 25 C increased only very slightly (average 0.02 acid value unit/month) during the first 17 months of storage to reach the value of 0.5. After 17 months of storage, acid value increased 5-fold faster reflecting accelerated triacylglycerol degradation (Fig. 1). After 21 months at 25 C in colored glass bottles, acid value of AAO reached the 0.8 limit, loosing its extra virgin label (Service de normalisation industrielle, 2003). When AAO was stored unprotected from sunlight at 25 C, increase in acid value began two months sooner and the 0.8 limit was reached after 19 months (Fig. 1). However, after two years, final acid value of AAO stored in clear or dark bottles was similar: 1.2 0.2. Stored at 40 C, AAO lost its extra virgin label after 18 months. Initial acid value of GPAO was much higher than that of MAO and AAO and already above the 0.8 limit (Table 1). When stored at 25 C in clear glass bottles, acid value of GPAO significantly and continuously increased (Fig. 1) whereas that of samples stored at
Initial Stored at 25 C Stored at 25 C Stored in clear glass in dark glass at 40 C MAO Schottenol 46 Spinasterol 40 D-7-avenasterol 5.5 Stigmasta-8,22-dien-3b-ol 5
46.5 40 4 3.5
46.5 39 4 3.5
45 37 4 3
AAO Schottenol 44 Spinasterol 42 D-7-avenasterol 4 Stigmasta-8,22-dien-3b-ol 3.5
44 42 4 3
44 41 3.5 3
44 39 3 3
GPAO Schottenol 44 Spinasterol 43 D-7-avenasterol 6 Stigmasta-8,22-dien-3b-ol 4
44 42 3 3
44 41 4 4
44 40 3 3
Fig. 1. Acid value of argan oil as a function of time (months). Oil samples were either sunlight-protected and stored at 25 C (black symbols), exposed to sunlight and stored at 25 C (white symbols), or sunlight-protected and stored at 40 C (grey symbols). Oil was prepared from fruits either peeled by goats (rhombs), mechanically peeled (triangles), or manually peeled (squares). Mean standard deviation of the values (three replicates) are presented.
S. Gharby et al. / LWT - Food Science and Technology 44 (2011) 1e8
5
Fig. 2. Peroxide value and K232 (bold line) of mechanically extracted argan oil (MAO) as a function of time (months). Samples were either protected from sunlight and stored at 25 C (rhombs), exposed to sunlight and stored at 25 C (triangles), or protected from sunlight and stored at 40 C (squares). Mean standard deviation of the values (three replicates) are presented.
the same temperature in dark bottles remained stable during 5 months (Fig. 1). At 40 C, GPAO acid value increased continuously, but less rapidly than the acid value of AAO at the same temperature (Fig. 1). Interestingly, the 2-year acid value profiles of MAO, AAO, and GPAO stored at 40 C or at 25 C and unprotected from sunlight were almost similar (Fig. 1). This result suggests that between 25 and 40 C, light is much more important than temperature to induce triacylglyceride oxidation in argan oil. 3.3.2. Peroxide value analysis Peroxides are the primary oxidation products that lead to rancidity. Therefore, their formation dramatically impacts oil shelf life and consumer acceptance. High temperature and light are two well-known factors generally promoting peroxide formation. In argan oil, the respective impact of these two factors is presently unknown. Initial peroxide value of MAO, AAO, and GPAO was found to be below 2 meq of O2/kg oil, well below the maximum peroxide value of 15 meq O2/kg oil defined for the extra virgin argan oil label (Service de normalisation industrielle, 2003). For MAO, AAO, as well as GPAO, storage at 5 C for two years led only to a very slight increase of the peroxide value (data not shown); the highest peroxide value of 3 meq O2/kg oil was observed for AAO. Peroxide value of MAO, AAO, and GPAO stored at 25 C or 40 C behaved differently. When stored at 25 C in dark or clear glass bottles, MAO peroxide value remained below the 15 meq O2/kg oil limit for two years. Accurate examination of the changes indicated that MAO peroxide value increased permanently over two years to reach the almost similar maximum values of 10.7 meq O2/kg oil for MAO protected from sunlight and 12 meq O2/kg oil for MAO exposed to sunlight (Fig. 2). However, the increase rate seems to be
light-dependent since the oxidation kinetic observed between light protected and unprotected samples was different (Fig. 2). Argan oil natural antioxidants are also likely to influence this kinetic. Storage of MAO in an oven at 40 C for two years led to peroxide values permanently higher than that observed when stored at 25 C. The final peroxide value was 16.3 meq O2/kg oil, whereas the limit of 15 meq O2/kg oil barrier was crossed after 21 months. Interestingly, the profiles of the peroxide values at 40 C and 25 C in dark bottles were similar. This likely means that a process identical, but amplified at 40 C, occurs in MAO protected from sunlight at 25 C or 40 C, and confirms that degradation and oxidative processes occurring in MAO are greatly accelerated under sunlight. AAO stored at 25 C in clear glass bottles crossed the 15 meq O2/ kg oil limit after 13 months. At the same temperature, 19 months were necessary when AAO was protected from sunlight. When AAO was stored at 25 C and exposed to sunlight, the peroxide value increased quite consistently over two years to reach 25.8 meq O2/kg oil after two years (Fig. 3). Sunlight protection led to lower peroxide values (14.4 meq O2/kg oil, and 18.3 meq O2/kg when stored at 25 C and 40 C; respectively) that were similar to that observed for MAO for corresponding storage conditions. However those values should be carefully handled since peroxide value underwent large fluctuations over two years. Such phenomenon was not observed for MAO possibly suggesting the occurrence for AAO of multiple secondary oxidation processes that did not occur in MAO and hence that could be related to the different content in minor components. With regards to the peroxide value, GPAO satisfied the virgin label requirements for 13 and 15 months when stored at 25 C in clear or dark glass bottles; respectively. When GPAO was stored at 25 C in clear glass bottles, peroxide value was at its highest
Fig. 3. Peroxide value and K232 (bold line) of artisanally extracted argan oil (AAO) as a function of time (months). Samples were either protected from sunlight and stored at 25 C (rhombs), exposed to sunlight and stored at 25 C (triangles), or protected from sunlight and stored at 40 C (squares). Mean standard deviation of the values (three replicates) are presented.
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Fig. 4. Peroxide value and K232 (bold line) of argan oil prepared from goat-peeled fruit (GPAO) as a function of time (months). Samples were either protected from sunlight and stored at 25 C (rhombs), exposed to sunlight and stored at 25 C (triangles), or protected from sunlight and stored at 40 C (squares). Mean standard deviation of the values (three replicates) are presented.
(19.8 meq O2/kg oil) after 15 months. Then, it decreased to reach the low value of 6.9 meq O2/kg oil after two years. This phenomenon was amplified at 40 C (Fig. 4). At 25 C and protected from sunlight, AAO and GPAO peroxide values behaved globally similarly. Therefore, as already observed from the acid value study, over two years, light also appears as the major parameter promoting hydroperoxide formation in all types of argan oil, elevated temperature favoring only secondary oxidation product formation. 3.3.3. K232 analysis Primary oxidation product formation can also be monitored by measuring specific extinction at 232 nm (K232). High quality argan oil should present a K232 lower than 2.5 (Service de normalisation industrielle, 2003). During two years of storage at 5 C, K232 of MAO, AAO and GPAO remained practically constant (initial value 1.06, 1.24, and 1.28 vs final value 1.27, 1.29, and 1.52 for MAO, AAO, and GPAO; respectively) as expected from the results of the peroxide value study. When MAO was stored at higher temperature, K232 was observed between 1.6 and 2 after 2 years. When stored at 25 C in clear or dark glass bottles, K232 and peroxide value evolved in a similar way (Fig. 2), suggesting the low incidence of secondary oxidative product formation. During storage at 40 C, although peroxide value increased swiftly between months 11 and 17, K232 absorption remained quite stable during this period. This strongly suggests the occurrence at 40 C of multiple, complex, and not fully identified oxidative processes for which the involvement of phospholipids can be eliminated since a similar behavior was also observed for AAO and GPAO (Figs. 3 and 4). Whereas MAO had K232 between 1.6 and 2 after two years, AAO K232 was between 1.8 and 3 after the same period of time. AAO K232 crossed the 2.5 barrier after 11 and 17 months of storage at 25 C in dark and clear glass bottles; respectively (Fig. 3). Surprisingly, during storage 40 C, K232 of AAO never reached the 2.5 value even though observed peroxide value reflected the occurrence of intense oxidative processes. Consequently, in that case, a direct correlation between peroxide value and K232 was uneasy to establish, likely due to multiple and concomitant oxidation processes favored by temperature. Finally, for GPAO, K232 after two years was between 1.6 and 2.4. K232 absorption remained surprisingly stable when GPAO was stored at 25 C in dark bottles. When stored at 25 C in clear bottles, a good correlation was observed between K232 and peroxide value, both indexes decreasing after 15 or 17 months (Fig. 4). During storage at 40 C, K232 and peroxide value increased simultaneously until month 17 but K232 remained stable although peroxide value dramatically plummeted after this month.
3.3.4. K270 study Carbonyl (aldehyde and ketone) compounds are the most abundant secondary oxidation products formed in edible oils. Their formation is known to be accelerated by elevated temperature and metal traces (Choe & Min, 2006). UV absorption at l 270 nm (K270) is one of the markers used to follow secondary oxidation formation. Moroccan regulation has set the maximum value for K270 at 0.35 (Service de normalisation industrielle, 2003). Overall, K270 values did not significantly changed over the 2 years. Initial values are given in Table 1. That argan oil samples stored at 5 C over 2 years displayed stable K270 was not surprising. That this trend also occurred for oil samples stored at higher temperature was unexpected. Final K270 values for MAO were 0.24, and 0.31 when samples were stored at 25 C, and 40 C, respectively. For GPAO, final values were in the same range for the three storage conditions. Only K270 of AAO stored at 40 C crossed the limit value of 0.35 after 17 months to end up at 0.39 0.05. Independently of the sunlight protection, the final value of AAO stored at 25 C was 0.21 0.05. In summary, K232 and peroxide value depict the formation of primary oxidation products. The apparent sample-dependent correlation observed between K232 and peroxide value supports the idea of different ratio of hydroperoxides depending on the type of argan oil. Decomposition of these hydroperoxides into secondary oxidation products can be monitored by K270 examination. Our results show that hydroperoxides formed in the three types of
Table 6 Rancimat induction period (hrs) at 110 C of argan oil prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) and stored at 5 C, 25 C (clear or dark glass bottles), and 40 C for up to 2 years. Mean standard deviation of the values (five replicates) are presented. 6 Months 12 Months 18 Months
Final
MAO Stored Stored Stored Stored
at at at at
5 C 25 C in clear glass 25 C in dark glass 40 C
32 28 29 28
1 1 1 1
30 28 28 27
1 1 1 1
31 26 27 26
1 1 1 1
30 25 27 24
1 1 1 1
AAO Stored Stored Stored Stored
at at at at
5 C 25 C in clear glass 25 C in dark glass 40 C
13 11 13 10
0.5 0.5 0.5 0.5
12 11 12 10
0.5 0.5 0.5 0.5
13 10 11 10
0.5 0.5 0.5 0.5
14 9 10 8
0.5 0.5 0.5 0.5
GPAO Stored Stored Stored Stored
at at at at
5 C 25 C in clear glass 25 C in dark glass 40 C
15 13 15 12
0.5 0.5 0.5 0.5
14 12 13 12
0.5 0.5 0.5 0.5
15 11 12 11
0.5 0.5 0.5 0.5
15 10 12 9
0.5 0.5 0.5 0.5
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Table 7 Saponification value, iodine index, and b-carotene level (initial and final) of argan oil samples prepared from argan kernels pressed 1) mechanically (MAO), 2) artisanally pressed (AAO), or 3) artisanally and obtained from goat-peeled fruit (GPAO) stored for 2 years at 5 C, 25 C in clear or dark glass, and at 40 C. Mean standard deviation of the values (five replicates) are presented. Stored at 5 C
Stored at 25 C in clear glass
Stored at 25 C in dark glass
Stored at 40 C
Saponification value (mg of KOH/g of oil) MAO 189.5 0.5 AAO 192.6 0.5 GPAO 190.6 0.4
189.7 0.2 192.7 0.5 192.5 0.5
190.8 0.5 193 0.5 191.7 0.6
190.6 0.4 193.2 0.2 191 0.5
194 0.1 193.9 0.5 193.4 0.6
Iodine index (g of I2/100 g of oil) MAO 97.7 0.1 AAO 102.4 0.5 GPAO 96.8 0.5
96.9 0.5 101.2 0.5 95.7 0.5
96.7 0.5 99.4 0.5 94.5 0.5
96.7 0.5 99.4 0.6 96.4 0.5
95.9 0.5 98.3 0.4 95.6 0.5
18.8 0.5 17.9 0.5 15.4 0.5
17 0.5 7.1 0.5 6.6 4
17.4 0.5 10.1 0.5 10.2 5
17 0.5 11.3 0.5 15.4 5
Initial
b-Carotene level (mg/kg) MAO AAO GPAO
20.7 0.5 18 0.5 17.5 0.5
argan oil decompose to unsaturated secondary oxidation products, and that MAO presents the slowest decomposition rate. In GPAO, the oxidative profile is more complex. Keeping in mind that MAO presents a highly homogeneous chemical composition, likely induced by its highly homogeneous geographical origin, these observations are of the utmost importance, considering the negative influence of secondary oxidation products on oil taste and smell, from an organoleptic standpoint. 3.3.5. Rancimat study Then we investigated the oil oxidative stability by measuring every 6 months the rancimat induction period at 110 C of MAO, AAO and GPAO stored in our evaluated conditions. Results are reported Table 6. When oil samples were stored at 5 C, rancimat induction period did not significantly vary over two years. Amazingly, at storage temperatures above 5 C, rancimat induction period of each type of oil decreased during the first 6 months then remained almost unchanged during the last eighteen months. MAO displayed by far the longest induction period confirming the good preservation properties of this type of oil. Rancimat induction time of oil samples stored at 25 C and exposed to sunlight was found to be slightly but significantly shorter than that of oil samples stored at the same temperature but protected from sunlight. This result is consistent with our previous observations that indicate the occurrence of a slower oxidative process in argan oil samples protected from sunlight. Storage of argan oil at 40 C for 2 years led to a reduction of the rancimat induction period almost similar to that observed for argan oil stored at 25 C and exposed from sunlight. 3.3.6. Miscealenous analyses Finally, we also decided to analyze some of the physicochemical parameters of our oil samples after two years in order to possibly detect variations affecting its pharmacologically essential components. Because most of argan oil therapeutic properties are linked to its high unsaturated fatty acid content, we determined several parameters including the iodine index, saponification value, and fatty acid composition of every oil samples after 2 years (Tables 3e5, 7). Concerning the saponification value, the largest variation was observed for samples stored at 40 C but the saponification value calculated after 2 years was still satisfying the official norm (Service de normalisation industrielle, 2003). Over two years, iodine index underwent a minor reduction due to primary oxidation but, for all samples it was consistently found between 91 and 110 as required by the official norm (Service de normalisation industrielle, 2003). Additionally, we also analyzed the b-carotene content of our oil samples since b-carotene actively participates in
oleic-rich oil protection under autooxidative and photooxidative processes (Goulson & Warthesen, 1999). While b-carotene level in MAO remained stable over two years, a strong decrease was observed for AAO stored at 25 C in clear glass bottles likely due to intense oxidative reactions. Measurements for GPAO were inconclusive due to the color variation of the oil samples after two years. Concerning the fatty acid and sterol distribution in each oil samples, no significant changes were observed over two years. Results are listed Tables 3 and 4. Tocopherols possess antioxidative and anti free-radical properties. Therefore oil oxidative stability depends on changes occurring in tocopherol content during storage (Okogeri & Tasioula-Margari, 2002). Tocopherol high concentration in argan oil is not only essential for its preservation but also for its pharmacological activity (Khallouki et al., 2003). Storage of argan oil for two years at 25 C in clear glass bottles resulted in a dramatic decrease in tocopherol level for the three types of oil. Sunlight protection resulted in a reduced tocopherol lost that was nevertheless consequent for oil samples stored at 40 C. Individually considered, a-, b-, and d-tocopherol levels were almost divided by two after two years of storage in clear glass bottles or at 40 C. Only storage in dark-glass bottles allowed the preservation of a high g-tocopherol level (Table 5). 4. Conclusions Combined all together, our results designate light as the major factor involved in argan oil oxidation. After two years of storage at 25 C, MAO protected from sunlight displays several physicochemical properties and an oxidative induction period that remained similar to freshly prepared argan oil. MAO is the type of argan oil that is sold on the international market and a shelf life of two years can reasonably be recommended for such oil as long as it is protected from sunlight. Edible argan oil has a characteristic copper color that helps consumers to distinguish it rapidly from other oils. Therefore the use of colored glass bottles is unlikely to be easily accepted by a majority of consumers. Argan bottles are generally packed in cardboard box, such practice should be preserved since to help argan oil preservation. Acknowledgments This work was performed in the frame of “Projet arganier” and financially supported by “Agence de Développement Social” and EEC (#AR05A061P704). We thank Association Ibn Al Baytar, Lesieur-Cristal, cooperatives Targant (Ait Baha), Taitmatine (Tiout), and Tidzi cooperative for their support and assistance in this work.
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